Epoxy resins are widely used as a commercial adhesive where its use can be found in industrial and electrical applications, consumer applications, marine and aerospace applications. The strong bonding property is due to the three-dimensional cross-linked polymeric network formed during curing of the epoxy, usually by reaction between a curing agent and the epoxy groups to obtain a desirable cross-linking density. Curing catalysts can also be added to the mixture to further accelerate the cure (cure accelerator).
Traditionally the epoxy resin and curative are either stored separately until the time of reaction or stored in refrigeration after mixing to prevent undesirable premature curing of the resin. This, however, is inconvenient and energy consuming. It also does not resolve the dilemma of increased cure rate at the expense of storage life.
One solution to this problem is the use of latent, or in-situ generated, curatives. These are chemical species that are stored in a shielded or inert form during storage conditions and that do not initiate or catalyse the epoxy reaction in the composition until some stimulus, typically temperature, is applied to generate the active form. Using this strategy, a one-part, ambient-temperature-stable curable epoxy resin can be achieved with fast curing rate when desired without sacrificing storage life of the epoxy composition.
For example, U.S. Pat. No. 3,395,105 describes a curative encapsulated in a hardened protective colloid such as gelatin and while it is stable for storage at ambient temperature, the curing agent can be released by application of pressure for reaction with the epoxy resin.
EP 1,252,217 describes the use of ambient-temperature-stable, impermeable microcapsules with thermoplastic polymeric capsule walls to encapsulate curatives to generate storage-stable, one-part, curable epoxy resin systems, and the method of curing the adhesive by heating the system at 80° C. or above.
U.S. Pat. No. 4,711,936 describes the use of a complex formed between a cyclodextrin and a compound that is reactive with an epoxy group as a curing agent for an epoxy resin at room temperature and the method of curing epoxy resin by heating the mixture of the curing agent and the epoxy resin at 90° C. causing the complex to decompose.
U.S. Pat. No. 4,711,936 does not provide a detailed description of the complex prepared. It is noted that the coating compositions used in the worked examples make use of toluene. This solvent is known to be a guest for a cyclodextrin, and there is the risk that the curative is displaced by toluene prior to the heat treatment.
U.S. Pat. No. 4,711,936 shows that one example composition may be stored for up 3 days. However, there are no examples of compositions that are stored over a longer, commercially relevant time frame. The worked examples show that temperatures of greater than 100° C. are needed, and with the exception of one worked example, all the example compositions are heated to temperatures of 120° C. or more.
U.S. Pat. No. 4,711,936 suggests that lower decomplexation temperatures are possible, for example, at around 40° C. However this requires the use of the enzyme amylase to digest the cyclodextrin host, which complicates the curing process.
The present invention provides alternative epoxy compositions for use in the preparation of cured epoxy resins. Such compositions allow for activation of a curing composition at a broad range of temperatures, including moderate temperatures, and without the need for the use of enzymes.